The present invention relates to spectroscopy, and more particularly, to rotating disk electrode spectroscopy.
One common method of spectrometric oil analysis involves use of optical emission spectroscopy for detecting and quantifying the presence of elements in the tested sample. Optical emission spectroscopy is based on the fact that each element has a unique atomic structure. When subjected to the addition of energy, each element emits light at characteristic wavelength(s), which emission is seen as spectral lines and are a unique signature for such element. The intensity of the emitted light is proportional to the quantity of the element present in the sample. Thus the presence and quantity of the element can be detected and indicated. If non-conflicting signatures are selected for a plurality of elements, then a plurality of elements can be detected in a single test.
The typical optical emission spectrometer includes a sample of oil input, an excitation source for exciting the elements in the sample into a transmission state, an optical system to isolate emissions into discrete wavelengths, and a readout system. The typical excitation source is an electric discharge, in the form of an arc or spark. Energy from the source is imparted to the sample. Typically, two electrodes form a capacitatively-driven spark gap and the oil is carried into the gap. Upon discharge, a portion of the oil is rapidly heated and vaporizes into a plasma, i.e., a hot, highly ionized gas which emits intense light. The light given off as a result of this energization contains emissions from all of the elements in the sample. The optical system identifies individual signature spectra and the readout communicates the identified spectra and the intensity thereof.
For some condition monitoring applications, such as for reciprocating engines, in which abnormal wear often tends to proceed gradually, with many fine wear particles being generated concomitantly with large particles, spectroscopic oil analysis is quite adequate. However, for other applications, such as for highly loaded rolling element bearings, as are used in military aircraft gas turbines, fatigue failures generate only rather large particles as surface spalling begins. These surface defects then lead to further stress concentration in the bearing subsurface causing rapid surface breakdown and bearing failure. Conventional spectrometric oil analysis may not give adequate advanced warning for this type of failure.
A special type of optical emission spectrometer, called a Rotating Disk Electrode (RDE) spectrometer, is used by many military and commercial labs performing Spectrometric oil analysis. An inexpensive annular carbon disk electrode (called a "rotrode") is pressed onto the end of a rotatable shaft. A sample of lubricating oil is loaded into a sample cap and positioned so that the bottom of the carbon disk can be rotated through the oil sample. A capacitative spark gap is formed between the top of the disk and the tip of a carbon rod electrode. Oil that is carried by the disk from the cap to the tip of the rod electrode is vaporized, dissociated and excited to form a plasma. As in conventional optical emission spectroscopy, the wavelengths of light emitted during this burning are characteristic of the elements present in the oil sample; the light is directed to a spectrometer optic where the characteristic wavelengths of interest are quantified. A new carbon disk rotrode is used for each sample.
A technique called ferrography was developed in the early 1970's partly to address the inability of conventional optical emission spectroscopy to detect large particles in used lubricating oil samples. Ferrography is a magnetic separation technique that deposits wear particles on a glass substrate (called a ferrogram) for subsequent microscopic examination. Magnetic particles, i.e., ferrous particles, are deposited in an orderly fashion according to size. Other wear particles and contaminants are codeposited in a random fashion. The presence of codeposited contaminants may be detected, but they are not readily quantified.
A so-called Direct Reading or DR ferrograph was invented in the mid-1970's to make ferrography more quantitative. DR ferrography produces two readings which quantify the concentration of large ferrous particles and the concentration of small ferrous particles in an oil sample. The DR ferrograph is nonresponsive to nonferrous materials such as lead, tin, copper, aluminum, etc.
It is therefore an object of the present invention to provide a simple method of large particle measurement in a used oil sample for early failure detection/prediction in a condition monitoring/predictive maintenance program.
Conventional spectroscopy, including RDE spectroscopy, detects and quantifies small particle contaminants, generally below 1-5 micrometers, but is relatively insensitive to larger particles. Conventional ferrography, including the DR ferrograph, can quantify large particle concentration of contaminants in an oil sample, but is essentially limited to ferromagnetic materials.
It is therefore another object of the present invention to provide an improved RDE method and apparatus for spectrometric analysis of used oils for condition monitoring/predictive maintenance without the size or metal-type limitations of the prior art.